1. Field of the Invention
The present invention relates to a wire saw slicing apparatus and a slicing method for cutting off or slicing a workpiece, such as a silicon semiconductor single crystal ingot, into wafer-like slices with no surface irregularities and kerf loss involved and with good yield resulting therefrom.
2. Description of the Related Art
Wire saws are used for cutting off or slicing a workpiece, such as a semiconductor single crystal ingot, into wafers by reciprocating a plurality of parallel regularly spaced lines of wire in the axial direction while feeding the workpiece relatively to the wire along a linear path to force the workpiece against the wire, with an abrasive-containing working fluid continuously supplied to a contact area between the workpiece and the wire lines. The wire saws are advantageous in that a multiplicity of wafers of the same thickness can be produced at one time. A slicing time required for each slicing run or operation is about 6 hours for a silicon semiconductor ingot having a diameter of 5 inches. In an alternative arrangement, the wire, like the workpiece in the foregoing arrangement, is fed relatively to the workpiece along a linear path to force the wire against the workpiece. In a description given below, only the first-mentioned arrangement incorporating a linear movement of the workpiece (in a downward direction, in general) will be described. It is obvious, however, that the second-mentioned arrangement incorporating a linear movement of the wire can be applied to in the same manner as the first-mentioned arrangement.
The sliced wafers ideally have the shape of a completely flat disk. In reality, however, their shape deviates from the ideal shape and are most likely to have a deformation called "sori" (warp or bow) observed, respectively, as a concave or a convex and an S-shaped deformation having a relatively large periodicity or radius of curvature, or a deformation called "undulations (waviness)" appearing as surface irregularities having a periodicity of several millimeters to several tens millimeters. The deformation of "sori" (warp or bow) and the deformation of undulations (waviness) cannot be removed by a subsequent wafer-finishing process such as lapping process because the wafers undergo elastic deformation under loads exerted thereon during the lapping process. In view of progressive microminiaturization of the electronic circuit, the deformation of "sori" (warp or bow) and undulations (waviness) of the as sliced wafers affects the yield of semiconductor devices formed on the wafers.
The deformation of "sori" (warp or bow) is generated due to variations of pitch of a wire wound around main rollers of the wire saw slicing machine, which are caused due to thermal expansion of the main rollers under frictional heat generated during the slicing process. To deal with this problem, various means for cooling the main rollers during the slicing process have been proposed.
The proposed method is able to preclude the generation of the deformation of "sori" (warp or bow) by cooling the main rollers for keeping the constant wire pitch. However, the deformation of undulations (waviness) on the as-sliced wafer cannot be avoided by the proposed method because the deformation of undulations (waviness) is considered to be generated under the effect of variations of feed speed of the workpiece relative to the wire taken in combination with temporary stops at opposite ends of reciprocating movement of the wire where the wire changes its direction of movement.